The invention relates to a method for protecting a bank-note or identification document on which a number is placed identifying the individual bank-note or identification document.
The fact that all kinds of copying techniques have become available to the public, means that it is becoming increasingly important to protect bank-notes and documents permitting the identification of persons. Counterfeiting must become increasingly difficult, and protection must always be ahead of the equipment available to the counterfeiter. Bank-notes and passports or other identification documents always have a number by means of which they can be recognized as unique notes or documents.
The object of the invention is to provide a method of the type mentioned in the preamble which leads to bank-notes or identification documents whose abovementioned unique number, or part thereof, is very difficult to counterfeit.
According to the invention, the method referred to in the preamble is characterized in that at least a part of said number is repeated at another place of the bank-note or identification document by reducing the thickness of the document locally by means of laser etching.
Laser etching, which is carried out by local removal of, for example, slightly less than half the thickness of the paper or plastic, clearly links the identification number or part thereof to the paper substrate. By local thinning of the paper or paper-thin plastic, security which is linked to the identification number is provided.
No hologram, film or kinogram image is therefore added, but only substrate material is removed to a certain depth.
As a consequence of laser etching the part of the document having the locally reduced thickness, gets a brown colour, most likely caused by the fact that the energy rich laser beam burns the substrate a little bit. The brown coloured marks, which are dark with respect to the surrounding substrate, do not have the appearance of a water mark. Comparing the printed marks with the repeated marks becomes difficult.
By exposing the part of the document on which the repeated part of the identification number has been placed, to bleaching or decolouring, the brown colour vanishes. A pre-treatment with a bleaching or decolouring agent, for instance by an impregnated pad, may take place immediately after the laser etching.
Oxidizing bleaching agents are for instance peroxides, hypochlorides, persalts such as potassium permanganate (KNMO4), potassium persulphate (K2S2O8), sodium perborate (Na2BO3), sodium percarbonate (Na2CO4) and ozone (O3). It is not excluded that the substrate itself comprises an oxidizing bleaching agent, in which case the treatment with a bleaching agent immediately after the laser edging can be omitted.
The invention also relates to a bank-note or other identification document on which an identification number has been placed, at least part of which is repeated by means of the abovementioned method.
EP-A-0388090 discloses a bank-note or such like security document having a look through figure provided in a region of the bank-note having a substantial uniform transparency which is more transparent than a majority of the remainder of the bank-note. On the back of the bank-note a solid figurine is printed in register with said look-through figure. Another solid figurine is printed elsewhere on the bank-note. There is no indication that a part of the individual bank-note number is repeated at another place of the bank-note by reducing thickness of the document by means of laser etching.
DE-A-3634098 relates to a method for making trueness features on bank-notes or the like by laser etching. The trueness features on all the bank-notes are the same and there is no talk of repeating at least part of the individual bank-note number at another place of the bank-notes.
The invention will now be explained further with reference to the FIGURE. This FIGURE shows a copy of a bank-note, on a slightly enlarged scale.